In the production of poloyolefins in fluidized bed reactors, a long standing problem has been the loss of unreacted monomers and other components of the fluidizing gas as a consequence of removing the solid product from the reactor. The polymer product is in the form of small particles and is generally removed by passing them, usually by positive gas pressure transfer assisted by gravity, while still more or less entrained in, or at least surrounded by, the gaseous atmosphere predominating in the reactor. Regardless of the system of valves which causes the flow of solid product to the desired destination, the gas leaving the reactor along with the product generally contains a significant volume of unreacted monomer. This unreacted monomer represents an economic loss in at least two ways--it is no longer in the reactor where it can form the desired product, and it represents an environmental and safety issue which must be dealt with. The fluidizing gas may include inert gases used, for example, to control the dew point of the recycle gas. This also represents an economic loss. It is therefore desirable to minimize the amount of gas accompanying the product as it leaves the reactor. There is also a secondary effect, arising from the care taken to minimize the removal of gas with the finished product--the systems developed to assure only a minimal loss of gas tend to retard the removal of solid product, which can cause the entire process to be dependent on the rate of product removal rather than the otherwise possible rate of production. It is highly undesirable for the production rate of a large, expensive reactor to be limited by the product removal system.
In Aronson's U.S. Pat. No. 4,621,952, it is pointed out that the original Union Carbide fluidized bed process for making polyolefins included a gas lock zone associated with the product discharge train. Unreacted monomer accompanying the product resin was vented and recycled back to the reactor by compression. See U.S. Pat. Nos. 4,003,712, 4,032,391, 4,255,542, and 4,302,565. The Aronson '952 patent describes the use of a settling zone to fill a vessel with as much solid material as is practical, thus minimizing the amount of gas in the settling zone or vessel before it is sealed off by appropriate valving from the rest of the system. It also describes a transfer zone downstream from the settling zone. While discharging to the transfer zone, the settling zone reaches an intermediate pressure which is then preserved and can be increased after the settling zone is emptied by connection to another settling zone containing higher pressure gas, to reduce the amount of gas that can enter the settling zone from the reactor in the next cycle.
In U.S. Pat. No. 4,535,134, the loss of gaseous monomer during solid product removal from a horizontal reactor is reduced by controlling the powder level in a receiving container.